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Pickens addressed reporters on the sidelines of a hedge fund conference in Las Vegas on Wednesday

Las Vegas, Nevada (CNN)Top Republican donors may be taking their time flocking to Donald Trump, but a prominent billionaire is on board: energy and hedge fund magnate T. Boone Pickens.

Pickens, speaking to reporters on the sidelines of a hedge fund conference in Las Vegas, said he talked to Trump Monday and confirmed to the New York billionaire and presumptive nominee he'd support his candidacy.

That's no small thing for Trump, whose presumptive position atop the GOP ticket has brought a new reality to his plate: the need to help raise hundreds of millions of dollars for his campaign and others across the Republican Party.

Pickens, always a top target for Republican fundraisers, regularly sends big checks to Republican causes and candidates in Washington, from members of Congress and presidential campaigns to various state parties. He wasn't initially on board with Trump, however.

"I didn't think he was serious," Pickens said Wednesday on the sidelines of the SALT Conference, which is considered one of the hedge fund industry's premiere conferences. Pickens was an early backer -- to the tune of $100,000 -- of former Florida Gov. Jeb Bush. He then looked to back Carly Fiorina and Ben Carson -- something Trump didn't hesitate to mention during the phone call this week between the two, Pickens said.

Pickens said he explained his shifting support simply, "You knocked 'em all out," he said he told Trump of the other candidates. That didn't keep him from ribbing his chosen candidate on a panel with fellow billionaire Sam Zell Wednesday morning.

"Donald always overestimates how successful he is," Pickens said.

"No s---," Zell quickly responded. Still, Pickens said, he was "willing to take a chance" on Trump given his frustration with other politicians, adding that he believed Trump was "smart enough that he's gonna get help" on policy and campaign issues.

Trump's willingness to ensure top GOP donors not only continue to give, but tell their friends to do the same, has been an open question among senior party officials. Trump's advisers have held meetings with top Republican National Committee officials to hash out a joint fundraising agreement -- a touchy subject for a candidate who prided himself throughout the primary of "self-funding."

Trump has acknowledged the need to tap into the Republican donor money spigot, if for no other reason than to help down ballot candidates. But the reality facing his campaign is he will likely need plenty of outside money for his own campaign. He's started building a finance operation, led by Steve Mnuchin -- a finance executive little known in the GOP donor community -- and RNC officials say they have confidence a more traditional presidential fundraising mechanism will be in place soon.

Pickens declined to detail how much he would be willing to donate to Trump's effort and Jay Rosser, an adviser to Pickens, said it's too early to know whether he will also help fund the super PAC supporting Trump. But he did add credence to something Trump's advisers have been telling worried GOP officials. Trump's Rolodex is real, and extensive.

Pickens said that while he's never done business with Trump, they've known one another for more than 30 years. He also said he'd now be more than willing to help convince other top business leaders to come aboard.